Precambrian Life and Cambrian Revolution

Abstract

The Precambrian or Cryptozoic Eon represents about 88% of the Earth’s geologic time. It is subdivided into Hadean (4543–4000 mya), Archean (4000–2500 mya), and Proterozoic (2500–541 mya) eons. Life originated on Earth according to the theory of organic revolution in the mid-oceanic ridges through a sequence of chemical reactions between carbon, hydrogen, oxygen, and nitrogen under electrical charge to produce amino acids that joined together to form proteins and nucleic acids, and these must be transformed into living, self-replicating structures, in oxygen-free environment at the Late Hadean to Early Archean time. The Precambrian in general is distinguished by the poverty of its life compared to the succeeding Phanerozoic. Furthermore, the Archean is greatly poorer than the Proterozoic. Archean life witnessed the change from the abiologic evolution to biologic evolution. All Archean life was pelagic prokaryotes including methanogens, bacteria, and cyanobacteria (blue-green algae). The oldest known fossils on Earth were recorded in the chert of the banded iron formations in the greenstone belt terranes in Canada, Greenland, Australia, and South Africa. The oldest Archean life records are organic carbon and graphite preserved as microbial mats in stromatolites included in the greenstone belt of Nuvvuagittuq in Quebec, Canada, and Isua in southwest Greenland dated at about 3700–3800 mya. The Archean oldest cellular life is recorded in Fig Tree Formation or Group in South Africa and the North Pole area in Western Australia at about 3500 mya. Methanogens are suggested to maybe as a link to the earliest forms of life. The prokaryote’s fossil records of Proterozoic are comparatively abundant, relative to that of the Archean. The Gunflint Chert algae from the Gunflint Iron Formation of Ontario, Canada, is the first Precambrian microflora to be described. It is dated to about 2 billion years and represents the evidence for diversity of prokaryotic life at that time. It is diverse and includes thread-like, rod-like, and spheroidal, star-shaped, and umbrella-shaped species identical in form to present-day bacteria and blue-green algae. Pelagic single-celled eukaryotic acritarchs appeared in about 1500 mya. Bitter Springs fauna in Australia gives evidence for diversification of eukaryotes and evolution of sexual reproduction. The Ediacaran fauna is soft-bodied with calcareophosphatic shells appeared in the Late Proterozoic Ediacaran Period between 635 and 541 Mya after the Cryogenian glaciation. In Egypt, an Ediacaran algal microfloral assemblage was described from the Igla Formation, Hammamat Group, cropping out in Wadi Igla, west Marsa Alam, Eastern Desert. This assemblage includes seven species related to phylum Cyanophyta and one species phylum Pyrrophyta. These in addition to microphtolites (oncolites) of cyanobacterial growth and other unnamed forms of probable algal affinities have also been reported. The transition from Precambrian at the end of the Ediacaran Period with soft-bodied fossils to Cambrian with appearance of the first assemblages of shelly faunas is known as the Cambrian Explosion. This major event in Earth’s history spans about 13–25 million years after 541 mya and led to the emergence of most modern metazoan phyla. This event is passed in three stages: low diversity, moderate diversity (Tommotian Stage), and high diversity (Atdabanian Stage). The last stage witnessed the first appearance of Trilobites. In Egypt, a rare microfossil assemblage including some archaeocyathids, sponge spicules, and the algal Bactrophycus oblongum Zhang was recorded in the Igla Formation, Hammamat Group, cropping out in Wadi Semna, near Safaga, Eastern Desert. These in addition to seven agglutinated foraminiferal species following four genera recorded from the same Igla Formation in Wadi Semna may indicate a Lower Cambrian age for that formation in Wadi Semna. The Araba Formation is the oldest Paleozoic sedimentary unit exposed in southwest Sinai and northern part of the Eastern Desert. The lower part of the Araba Formation includes a limestone bed with body fossils of stromatolites and archaeocyathid elements of probable Early Cambrian age. The Araba Formation is found ichnofossil bearing, and about 20 species of these trace fossil species were recorded from the middle unit of the Araba Formation and ensured its Lower Cambrian age. The Burgess Shale fauna is a middle Cambrian exceptional assemblage of more than 170 species representing at least eight known phyla of animals of soft-bodied and biomineralized organisms. It affords a profoundly informative picture for the Cambrian post-explosion world. All oxygen in the Earth’s atmosphere was produced in Early Proterozoic time by photosynthesis of the cyanobacteria. This released oxygen in the atmosphere and the shallow oceans experienced an oxygen rise leading to the oxygen revolution at approximately 2400–2000 Mya which killed major part of the living prokaryotes.